I was stuck in Diamond II for three straight days. Nothing was working at all for me. Then I got the idea of replacing the staple Aurelian Merchant that was in my Xenan Killers deck. I did this for a few reasons. In the current meta the decks seeing the most play right now (at least in my experience) are one variety or another of time midrange, Scream, and Temporal Control.
The Xenan Killers decktype in general is competitive against Combrei decks that run
Sword of Unity thanks to
Xenan Initiation and Vara. Aurelian Merchant doesn't provide much board presence against other midrange decks, and it doesn't provide an ideal tool to shut control down.
Kerendon Merchant is both an excellent blocker and a sneaky-good attacker because people don't want to trade their 4-drop Titans and Varas for a 2/1. It provides solid, fast removal to slow Scream down while you establish your stronger board position. It also provides access to a suite of market removal and most importantly
Azindel's Gift, which is the strongest answer against Temporal Control.
The market is full of pretty common shadow cards. The weakest one in my experience is the
Azindel, Revealed: the game rarely goes so long that you need him to stabilize. I'm considering replacing him with a Banner or a
Dark Return.
Suffice to say, I've gone 13-1 and waltzed to Masters with this deck so far.